The present application claims priority to Japanese Application(s) No(s). P2000-273956 filed Sep. 8, 2000, and P2000-392872 filed Dec. 25, 2000, which application(s) is/are incorporated herein by reference to the extent permitted by law.
The present invention relates to an alkaline battery suitable for use as a coin-type alkaline battery or a button-type alkaline battery of flat structure.
Coin-type or button-type alkaline batteries are used for small-sized electronic machines and equipment such as electronic wrist watches and portable electronic computers. As shown in FIG. 4, which is a schematic sectional view, they are constructed such that the open end of a cathode can 1 is sealed with an anode cup 3 with a gasket 2 interposed between them.
The anode cup 3 has its peripheral edge folded back as indicated by a fold 13 which has a U-shape cross section. The fold 13 holds the gasket 2, which is tightened inside by the open end of the cathode can 1, so that hermetical sealing is achieved.
The anode cup 3 is press-formed from a triple-layer laminate plate consisting of an outer surface layer 31 of nickel, a metal layer 32 of stainless steel (SUS), and a current collector layer 33 of copper.
The cathode can 1 holds a cathode mix 4 which contains silver oxide or manganese dioxide as a cathode active material. The anode cup 3 holds an anode mix 6 which contains mercury-free zinc or zinc alloy powder as an anode active material. The anode mix 6 is separated from the cathode mix 4 by a separator 5 and is filled with an alkaline electrolytic solution.
The above-mentioned anode mix 6 may be replaced by amalgamated zinc or zinc alloy powder in order to suppress evolution of hydrogen gas (H2) from zinc powder or zinc alloy powder or evolution of hydrogen gas (H2) from the current collector layer 33 of copper of the anode cup which comes into contact with zinc or zinc alloy powder through the alkaline electrolytic solution. Evolution of hydrogen gas results from the reaction which dissolves zinc or zinc powder in the alkaline electrolytic solution, thereby oxidizing zinc into zinc oxide.
This reaction is suppressed in the case where amalgamated zinc is used. The consequence is the avoidance of capacity deterioration due to hydrogen evolution and leakage and swelling of batteries due to increased internal pressure.
Recently, there is a trend toward avoiding the use of mercury in coin-type or button-type alkaline batteries as far as possible from the environmental point of view, and many research are being made for this purpose.
There have been proposed some methods of suppressing evolution of hydrogen gas from zinc or zinc alloy powder in alkaline electrolytic solution. One involves incorporation of zinc powder with a metal having a high hydrogen overpotential, and the other involves incorporation of the alkaline electrolytic solution with an inhibitor to suppress evolution of hydrogen.
However, none of them can completely suppress the evolution of hydrogen gas which results from zinc or zinc alloy powder coming into contact with the current collector through alkaline electrolytic solution.
It has been suggested that the evolution of hydrogen gas mentioned above is effectively suppressed by coating the copper current collector with any one of tin, indium, and bismuth or an alloy thereof, which has a higher hydrogen overpotential than copper. It has also be proposed that the copper surface of the current collector (anode cup) is coated by plating or the like. This coating, which is accomplished by electroless plating or barrel plating, gives the coating layer 30 shown in FIG. 5. The coating layer 30 is formed over the entire inner surface of the anode cup 3. In other words, the inner fold and the bottom of the fold of the anode cup are also coated with any one or more of tin (Sn), indium (In), and bismuth (Bi).
Incidentally, those corresponding parts in FIGS. 4 and 5 are given the same reference numerals to avoid duplicated explanation.
It has been found that the coating layer 30, which effectively suppresses the evolution of hydrogen gas, is more liable to cause the alkaline electrolytic solution to creep up than the copper layer of current collector. This creeping leads to the possibility of the electrolytic solution leaking out of the seal between the open end of the cathode can 1 and the anode cup 3 when pressure in the battery rises due to evolution of hydrogen gas for one reason or another.
One way to obviate the inconvenience is to form the coating layer 30 in a limited region on the inside of the anode cup 3 excluding the bottom 13a of the U-shaped fold of the anode cup 3 and the outer surface 13b of the U-shaped fold, as shown in FIG. 6.
Although the problem with creeping is solved as mentioned above, there still exists the possibility of hydrogen gas occurring when the coating layer 30 suffers defects such as pinholes, cracks, and contamination with impurities. Such defects permit zinc or zinc alloy to come into electrical contact with the copper of the current collector through the electrolytic solution. Thus this problem is not completely solved by the mercury-free anode structure.
Particularly, the possibility of hydrogen gas occurring is not eliminated in the case where the anode cup is made of a material which has previously been clad with the coating layer 30, because the coating layer 30 is subject to scratching and cracking or contamination with impurities before cladding.
In actual mass production, however, it is difficult to perform partial plating accurately in a limited region on the inside of the anode cup excluding the fold and the bottom of the fold. There is the possibility that the copper surface of the anode cup (current collector) is oxidized by the plating solution during washing.
Even though the metal to suppress the evolution of hydrogen gas (H2) does not exist on the fold and the bottom of the fold of the anode cup, the oxidized cupper surface of the anode cup (current collector) promotes the creeping up of the alkaline electrolytic solution and lowers the leakage resistance.
Difficulties are involved in applying to mass production the technology of preventing the evolution of hydrogen gas (H2) and suppressing the creeping up of the alkaline electrolytic solution.
For the above-mentioned reasons, no mercury-free alkaline batteries of coin type or button type have been put on the general market yet.
It is an object of the present invention to provide a highly reliable alkaline battery.
To achieve the above object, according to an aspect of the present invention, there is provided an alkaline battery constructed of a cathode can and an anode cup in such a way that an open end of the cathode can is sealed by the anode cup, with a gasket interposed between them, characterized in that the open end of the anode cup is folded back in U-shape along its periphery and the fold is tightened for hermetic sealing by the internal periphery of the open end of the cathode can, with the gasket interposed between them, the anode cup has a tin coating layer formed in a limited region on the inside thereof excluding the bottom of the U-shaped fold and the outer periphery of the fold, the cathode can contains the cathode mix which is silver oxide or manganese dioxide, or other metal oxide as the cathode active material incorporated with silver-nickelite (AgNiO2), the anode cup contains the anode mix which is mercury-free zinc or zinc alloy powder as the anode active material separated from the anode mix by a separator, and the anode mix is impregnated with an alkaline electrolytic solution.
Further, according to an another aspect of the present invention, there is provided an alkaline battery having a cathode can and an anode cup which are hermetically sealed, with a gasket interposed between them, the cathode can holding a cathode mix containing silver oxide or manganese dioxide or other metal oxide as a cathode active material, the anode cup holding an anode mix containing zinc or zinc alloy powder as an anode active material and having a peripheral fold and the bottom of the fold and a copper inside surface, the cathode mix being separated from the anode mix by a separator, and the anode mix being impregnated with an alkaline electrolytic solution, characterized in that a coating film of a metal or an alloy thereof having a higher hydrogen overpotential than copper is formed by dry process in a limited region on the inside surface of the anode cup excluding the fold and the bottom of the fold.